A variety of processes are used to recover viscous hydrocarbons, such as heavy crude oils and bitumen, from underground deposits. A problem associated with producing hydrocarbons from such deposits is that the hydrocarbons are too viscous to flow at commercially viable rates at temperatures and pressures present in the reservoir. In some cases, these deposits are mined using open-pit mining techniques to extract the hydrocarbon-bearing material for later processing to extract the hydrocarbons.
Over the past few years, improved techniques including thermal and solvent techniques have been developed to recover viscous hydrocarbons. Thermal techniques heat the reservoir fluids and rock to produce the heated, mobilized hydrocarbons from wells. One of the drawbacks to the thermal techniques is a large amount of energy (e.g., steam) is required to produce the crude oil or bitumen. For example, approximately 2.5 barrels of water converted to steam are required to produce one barrel of oil. This steam generation requires burning of fossil fuel (e.g., natural gas, coal) that results in undesirable green-house gas emissions.
One thermal method of recovering viscous hydrocarbons using two vertically spaced wells is known as a steam-assisted gravity drainage (SAGD) process. In the SAGD process, steam is pumped through an upper, horizontal injection well into a viscous hydrocarbon reservoir while the heated, mobilized hydrocarbons are produced from a lower, parallel, horizontal production well vertically spaced proximate to the injection well. The injection and production wells are typically located close to the bottom of the hydrocarbon deposits.
Prior approaches also utilize solvent to dilute the formation fluids for production of the diluted, mobilized hydrocarbons from wells. The solvent soaks into the formation fluids to dilute and reduce the viscosity of the heavy crude oil and bitumen. A vapor extraction (VAPEX) technique is similar to the SAGD process in terms of well arrangement and location, however, the technique differs in terms of the fluid injected, i.e., solvent vapor instead of steam. One of the drawbacks to the VAPEX technique is solvent penetration of heavy crude oil or bitumen is typically quite slow, which results in low production rates. Further, some portion of the solvent is lost in the reservoir, which increases the cost of production. In comparison to conventional SAGD, addition of the solvent to the steam in the SAGD process increases recovery and decreases energy requirements per barrel; however, amount of the steam used over time is unchanged given the increased recovery.
Therefore, a need exists for processes to recover heavy crude oil and bitumen that is energy efficient and provides an economical production rate.